PARAMETERS OF INUNDATION, RUN-UP AND DEPOSITION OF THE SOUTH ASIAN TSUNAMI OF 26 DECEMBER 2004 IN SOUTHEASTERN INDIA AND KENYA

The study of the 26 December 2004 South Asian tsunami deposits allows for the determination of tsunami run-up characteristics and depositional processes. Our data will contribute to recognizing ancient tsunami deposits in the rock record. This recognition is hampered by the poor preservation potential of the deposits in the commonly subaereal zones of run-up. Preservation is more probable in the marine realm, however, event beds may frequently not be identified as tsunami deposits because of the absence of uniquely indicative sedimentological features and the lack of physical continuity into run-up zone sediments. There is no tsunami facies model or any depositional model with predictive power in three dimensions.

We surveyed beaches north of Pondicherry, Tamil Nadu, India and in Kenya north of Malindi which were affected by the Sumatra tsunami of 26 December 2005, and sampled the tsunami deposits. In Tamil Nadu, the tsunami struck shortly after high tide at c. 9:05 am local time. Depending of the beach profile, the major tsunami wave covered a run-up distance of up to 600 m and had a run-up height of almost 6 m. Discolorations in the lower portions of palm trees indicate that the tsunami had an overland flow depth of over 3.5 m 100 m inland from the swash zone. Run-up distances and heights for two lesser waves following the major one could also be established. In Kenya, the tsunami hit the coast at 1:00 pm, shortly after low tide. Run-up distance on relatively steep beaches unprotected by reefs was c. 45 m, the run-up height 3 m, the flow depth was probably less than 1 m.

The sampled tsunami deposits commonly have erosive bases, the grain size distributions reflect the respective pre-tsunami shelf sediments. In Tamil Nadu, the deposits consist of coarse to medium sand and display a landward decrease in grain size and taper out c. 330 m from the shore. Minor cross bedding and dewatering structures are present. In Malindi, medium to fine sand together with oyster and gastropod shells >5 cm in diameter were transported to maximum run-up. Here, the deposits are sometimes weakly cross-bedded and rich in heavy minerals originally delivered to the sea by a nearby river. There do not seem to be obvious grain size trends. At all studied beaches, currents induced by the tsunami during run-up had minimum velocities of 200 cm/s.